Nanotechnology Spotlight

Improved molten air battery operates at lower temperatures

(Nanowerk Spotlight) One of the biggest obstacles that is holding back the wide-spread adoption of electric cars is the insufficient performance of batteries. They don't last very long, requiring a very dense network of charging stations, they are heavy, and they are expensive.

A new class of high-density, rechargeable batteries might change this picture by addressing the 'range anxiety' that is inherent to current electric vehicles by drastically increasing their battery capacity: molten air batteries have up to 50 times the storage capacity of lithium-ion batteries. These batteries reversibly use oxygen from the air to store energy via a molten salt and multiple electrons stored per molecule at the counter electrode. We introduced this concept to our readers in a previous Nanowerk Spotlight ("A new class of high-energy rechargeable batteries - molten air").

Unlike previous rechargeable molten batteries, the molten air battery is not burdened by the weight of the active chargeable cathode material. The rechargeable molten air electrode instead uses oxygen directly from the air to yield high battery capacity.

"We start with iron, carbon or vanadium boride for their ability to transfer multiple electrons," Stuart Licht, a professor of chemistry at George Washington University, explains to Nanowerk. "Molten air batteries made with these materials can store three, four and 11 electrons per molecule respectively, giving them 20 to 50 times the storage capacity of a lithium-ion battery, which is only able to store a maximum of one electron per molecule of lithium."

Licht and his team have now published a new study in the May 2, 2014 online edition of Journal of Materials Chemistry A ("A Low Temperature Iron Molten Air Battery"), in which they introduce a more EV (electric vehicle) compatible, i.e. lower temperature, version of their molten air battery, and demonstrate this with one version – iron molten air – of the battery.

Various molten air battery chemistries. (Image: Licht Group, University of Washington)

Previous versions of the molten air battery operated at temperatures of 700°C to 800°C, which is about the same level that is typically reached by internal combustion engines driven by gasoline. The refined version of the iron molten air battery introduced in this paper can operate at a working temperatures below 600°C, which makes it more compatible with EV applications.

Licht notes that, in addition to electric vehicles, these molten air batteries have a high capacity and potential for low cost – they function with inexpensive steel and nickel electrodes – for electric grid storage applications. The storage of energy for the electric grid increases the viability of renewable energy to provide continuous electricity when sunlight or wind is not available.

"The battery design is scalable and we are interested in moving the battery from laboratory to larger prototypes," he concludes.